Moving laser beam readers or laser scanners, as well as solid-state imaging systems or imaging readers, have both been used to electro-optically read one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, each having a row of bars and spaces spaced apart along one direction, and two-dimensional symbols, such as Code 49, which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol, as described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure for increasing the amount of data that can be represented or stored on a given amount of surface area is known as PDF417 and is described in U.S. Pat. No. 5,304,786.
Moving laser beam readers generally include a laser for emitting a laser beam, a focusing lens assembly for focusing the laser beam to form a beam spot having a certain size at a focal plane in a range of working distances, a scan component for repetitively scanning the beam spot across a target symbol in a scan pattern, for example, a scan line or a series of scan lines, across the symbol multiple times per second, e.g., forty times per second, a photodetector for detecting laser light reflected and/or scattered from the symbol and for converting the detected laser light into an analog electrical signal, and signal processing circuitry including a digitizer for digitizing the analog signal, and a microprocessor for decoding the digitized signal based upon a specific symbology used for the symbol.
The imaging reader includes a solid-state imager or sensor having an array of cells or photosensors that correspond to image elements or pixels in a field of view of the imager, an aiming light assembly having an aiming light source, e.g., an aiming laser, and an aiming lens for generating an aiming light pattern or mark on a target symbol prior to reading, an illuminating light assembly for illuminating the field of view with illumination light from an illumination light source, e.g., one or more light emitting diodes (LEDs), and an imaging lens assembly for capturing return ambient and/or illumination light scattered and/or reflected from the symbol being imaged over a range of working distances and for projecting the captured light onto the array. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing electronic signals corresponding to a one- or two-dimensional array of pixel information over the field of view.
It is therefore known to use the imager for capturing a monochrome image of the symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use the imager with multiple buried channels for capturing a full color image of the symbol as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
As advantageous as both types of electro-optical readers are in reading symbols, it is always desirable to enhance performance. Increasing the intensity or brightness of the laser beam of the laser in the moving laser beam reader will increase the working distance range, because there will be correspondingly more return light to detect from symbols that are further away from the moving laser beam reader. Similarly, increasing the intensity or brightness of the aiming laser in the imaging reader will increase performance, because the aiming pattern will be more visible to an operator, especially for symbols that are further away from the imaging reader.
However, increasing the laser beam intensity too much for either the laser in the moving beam reader or the aiming laser in the imaging reader may violate human eye exposure laser safety standard limits. For example, a class 2 laser is limited to an output power of 1 mW over a base time interval of 250 msec, and a class 1 laser is limited to an output power of 0.39 mW over a base time interval of 10 sec. The laser beam intensity cannot exceed these limits not only in normal operation, but also in the event of reader malfunction or failure of laser power control circuitry specifically provided in each reader to insure that these limits are never surpassed.
The known laser power control circuitry in such readers monitors the laser current in order to provide feedback about the output power of the laser beam. Also, an internal light detector, e.g., a semiconductor monitor photodiode, is typically mounted inside the laser adjacent a semiconductor laser chip, for monitoring the output power of the laser beam. A controller is operatively connected to the monitor photodiode, for controlling a monitored output power of the laser beam by deenergizing the laser when the monitored output power of the laser beam exceeds a safe power level limit.
As advantageous as the use of the monitor photodiode has been in monitoring laser power, recent changes in the manufacture of such lasers to decrease their cost have hindered their continued ready use in existing electro-optical readers. Heretofore, the cathode of the laser chip and the anode of the monitor photodiode were both grounded, thereby enabling the monitor photodiode to be reverse biased with a positive power supply readily on hand in such electro-optical readers. However, the recent manufacturing changes to reduce cost and to accommodate the use of such lasers in other industries, e.g., the digital video recording industry, have resulted in the cathodes of the laser chip and of the monitor photodiode to both be grounded. Now, in order to reverse bias the monitor photodiode to operate in a photoconductive mode, a negative power supply is needed, but it is not available in existing electro-optical readers.
Without a means to reverse bias the monitor photodiode, one might attempt to operate the monitor photodiode in a photovoltaic mode, i.e., without any external bias whatsoever. However, light incident on the monitor photodiode would then produce an unpredictable positive bias voltage across the monitor photodiode, as well as an unpredictable photodiode leakage current to flow through the monitor photodiode in opposition to a main photodiode current representative of the monitored output power of the laser beam. The photodiode leakage current corrupts the main photodiode current, and is also temperature-dependent.
Accordingly, there is a need for being able to take advantage of the low cost benefit of lasers in current manufacture, and to obtain them from multiple sources, even from other industries, and to use them in electro-optical readers without corrupting the main photodiode current representative of the monitored output power of the laser beam by the presence of photodiode leakage current.